To run NUnit, you need to add the following to the NUnit.exe.config file:
Under C:\Program Files\NUnit 2.5.9\bin\net-2.0\nunit.exe.config (replace version with whatever you have installed) add the following:
<!-- the below is WRONG! See here for why.
Also add the following in the <runtime> block:
The reason is that in .NET v3.5 and below, remote assemblies were loaded partially trusted. Starting with .NET v4, unless you add this directive then you will get a runtime exception.
Once you have done this, add another dll project and add a reference to NUnit.Framework.